1. Field of the Invention
The present invention relates to an electrode body for a solar cell with excellent heat resistance that can be used as a component of both an organic thin-film solar cell and a dye-sensitized solar cell, and a production method thereof. The present invention also relates to a solar cell with the electrode body.
2. Description of the Related Art
Organic solar cells, which can be roughly categorized into two types, organic thin-film solar cells and dye-sensitized solar cells, have the following advantages compared with silicon solar cells. The organic solar cells have no resource constraint, the production cost thereof can be curbed because of inexpensive raw materials and simple production processes, and they can be made lightweight and flexible.
An organic thin-film solar cell has a structure in which a photoelectric conversion layer with a hole transporter (p-type semiconductor) and an electron transporter (n-type semiconductor) is wedged between a positive electrode and a negative electrode. Generally, a transparent electrode, in which a vapor-deposited layer of semiconductive ceramics such as tin-doped indium oxide (ITO) and fluorine-doped tin oxide (FTO) is formed on the surface of a transparent substrate such as glass, is used as the positive electrode, and a metal electrode such as aluminum film and magnesium-silver alloy film, which has a smaller work function than ITO and FTO, is used as the negative electrode. When light is irradiated on the photoelectric conversion layer through the transparent electrode, an electron and a hole are formed in the photoelectric conversion layer, and the hole is transported to the positive electrode through the hole transporter, and the electron is transported to the negative electrode through the electron transporter, respectively, in isolation.
The performance of an organic thin-film solar cell is affected not only by the photoelectric conversion layer but the surface boundary between the positive electrode and the photoelectric conversion layer. Due to the poor smoothness and adhesiveness between the positive electrode and the photoelectric conversion layer, the transport efficiency of a hole from the photoelectric conversion layer to the positive electrode is decreased, which lowers the short-circuit current density of a solar cell and decreases the photoelectric conversion efficiency. To prevent this, a hole extraction layer composed of a conductive polymer layer with hole transportation capability is placed between the positive electrode and the photoelectric conversion layer. This hole extraction layer mainly has the function of smoothing the surface of the positive electrode and decreasing the interface resistance between the photoelectric conversion layer and the positive electrode.
As the hole extraction layer, a layer of polythiophene, especially a polystyrene sulfonate of poly(3,4-ethylenedioxythiophene) has been frequently used (hereinafter 3,4-ethylenedioxythiophene is referred to as “EDOT”, poly(3,4-ethylenedioxythiophene) as “PEDOT”, polystyrene sulfonic acid as “PSS”, and a polystyrene sulfonate of poly(3,4-ethylenedioxythiophene) as “PEDOT:PSS”). For example, Non-patent Document 1 (Solar Energy Materials & Solar Cells 94 (2010) 623-628) discloses an organic thin-film solar cell that is produced by forming a hole extraction layer by spin-coating an aqueous PEDOT:PSS dispersion on a positive electrode having an ITO layer on a glass substrate, and then forming a hole transporter layer consisting of copper-phthalocyanine, an electron transporter layer consisting of fullerene, a hole block layer consisting of a thin film of lithium fluoride, and a negative electrode consisting of an aluminum film by a vacuum deposition method in this order. This Document reports that asperity of the surface of the ITO electrode was remarkably improved by the PEDOT:PSS hole extraction layer, the transport efficiency of a hole from the photoelectric conversion layer to the positive electrode was remarkably improved, and as a result the short-circuit current density of the solar cell was greatly increased.
A dye-sensitized solar cell has a structure in which an electrolyte layer containing paired oxidized species and reduced species is wedged between a negative electrode with a semiconductor layer containing a pigment as a photosensitizer and a positive electrode with a catalyst layer to convert the oxidized species in the electrolyte layer to the reduced species. Generally, an electrode in which an oxide semiconductor layer supporting a pigment such as ruthenium complex is formed on the above-mentioned transparent electrode is used as a negative electrode and an electrode in which Pt is bonded on a substrate such as the above-mentioned transparent electrode or steel by a sputtering method or a vacuum deposition method is used as a positive electrode. When light is irradiated on the pigment of the semiconductor layer through the transparent electrode, the pigment absorbs light energy and becomes excited, and emits an electron toward the semiconductor. The emitted electron moves from the semiconductor layer to the transparent electrode, and further moves from the transparent electrode to the positive electrode via an external circuit. Then, by the action of the Pt catalyst layer of the positive electrode, the oxidized species (for example, I3−) in the electrolyte layer receives an electron from the positive electrode and is converted to the reduced species (for example, I−), and further, the reduced species (for example, I−) emits the electron toward the pigment and is converted to the oxidized species (for example, I3−).
The Pt catalyst layer of the positive electrode has a problem in that, though it has excellent catalytic activity to convert an oxidized species of an electrolyte layer into a reduced species, it is expensive and does not have enough durability against I− ions when water exists. Therefore, a conductive material as a substitute of the Pt catalyst layer has been hitherto considered, and a polythiophene layer, especially a PEDOT:PSS layer has been considered. For example, Non-patent Document 2 (Electrochemistry 71, No. 11 (2003) 944-946) reports the results of selecting an electrode with three types of conductive polymer layer, a PEDOT:PSS electrode, a polyaniline electrode and a polypyrrole electrode, evaluating a cyclic voltammogram in an electrolyte containing an I−/I3− redox pair and making a comparison with that of a Pt electrode. While the cyclic voltammogram of the Pt electrode clearly shows a reduction wave from I3− to I−, the cyclic voltammograms of the PEDOT:PSS electrode and the polypyrrole electrode hardly show a reduction wave from I3− to I−, and the cyclic voltammogram of the polyaniline electrode does not show an oxidation-reduction wave at all.